This paper investigates the mechanism that causes x-ray enhancement in high-Z foams for laser-driven soft x-ray sources. By simulation of one-dimensional radiation-hydrodynamics, it is found that the x-ray enhancement is mainly due to the effect that, in a foam target, shock wave compression significantly reduces the energy loss of hydrodynamic motion (kinetic energy). In a solid target this effect is negligible for its low compressibility. Expressions of kinetic energy reduction (ΔEk = Ek,solid − Ek,foam) are given to model the improvement of the laser-to-x-ray conversion efficiency. The ΔEk given by the model agrees with the simulation result with about a 15% error for foam density 0.07 – 0.3 g/cc and for laser intensity 0.4 × 1015–2.0 × 1015 W cm−2. The model indicates that the x-ray enhancement is more efficient with a lower foam density and higher laser intensity, which is also presented by the simulation results.